Development And Performance Research Of Self-humidifying Membrane Electrode Assembly For PEMFC

The proton exchange membrane fuel cell（PEMFC）has the advantages of high specific energy,environmental friendliness,and quick start-up at room temperature.It is considered to be one of the most promising new energy technologies in this century.However,for polymer membranes represented by perfluorosulfonic acid(Nafion^?),their proton transport capacity depends heavily on their degree of hydration.Therefore,the gas in the PEMFC system usually requires additional humidification to ensure good hydration of the Nafion^?polymer in the membrane and calalyst layer.At the same time,the excess water generated by gas humidification and electrode reaction must be discharged in time to avoid blocking the gas transmission channel and causing“fluid flooding”.Therefore,strict water management increases the complexity and cost of the PEMFC system and hinders the commercialization of PEMFC.Therefore,the development of self-humidifying membrane electrode assembly（MEA）to simplify water management is of great significance to the practical application of PEMFC.This paper starts from the perspective of calalyst layer modification,carry out self-humidifying MEA development and performance research by adopting the method of doping moisture-retaining functional materials in the calalyst layer and designing the water management area（WMA）of the calalyst layer.First,we proposed a strategy to develop MEA with self-humidification capability using covalent organic framework materials（COF）.COF supported by H₃PO₄ has good compatibility with Nafion ionomers,and its inherent-NH-,-PO₃H hydrophilic groups and abundant holes can be used as moisture absorbents in the PEMFC calalyst layer.In addition,the immobilized H₃PO₄ molecules can further retain water in the cavity,which can greatly increase the water retention rate of the calalyst layer under low humidity conditions.The experimental results show that the calalyst layer doping with COF improves the performance of PEMFC under low humidity.MEA shows excellent low humidity performance at 60℃and 38%relative humidity（RH）,and the maximum power density is as high as 582 m W cm^-2.It is 6 times higher than the performance of conventional electrodes under the same operating conditions.Further durability tests show that the doped COF material and modified MEA have good stability under low humidity operating conditions.Secondly,based on the previous work,we considered that direct doping of COF in the anode catalyst layer may have adverse effects on cell performance,so we optimized the addition ratio of COF and explored the impact of different electrode side modifications on performance.On the basis of not reducing the catalyst,reduce the amount of Nafion ionomer to optimize the MEA performance;at the same time,the anode,cathode,and anode and cathode are modified after determining the optimal addition ratio.We found that when the anode catalyst layer contains 15 wt.%H₃PO₄@SNW-1 and 15 wt.%Nafion ionomer,it has the best performance.The peak power density(690 m W cm^-2)under low humidity is even equivalent to the performance under 100%relative humidity,which is 9 times the power density of the traditional MEA under the same conditions,and is nearly 100 m W cm^-2 higher than the previous work.In addition,at 60 ^oC and fully humidified conditions,the peak power density of the optimized MEA is about 7%higher than that of the traditional MEA,reaching 745 m W cm^-2.After considering the effects of reaction kinetics and water evaporation,we recommend that the cell operating temperature be 50-60 ^oC.Finally,the preliminary 36h stability test and 10 days simulation conditions have once again proved the potential of the proposed MEA for practical applications in low-humidity environments.Finally,we developed a MEA with self-humidification capability by designing a calalyst layer WMA.WMA is prepared by spraying slurries with different properties（hydrophilic or hydrophobic）on the surface of the pre-prepared gas diffusion layer（GDL）according to the template,and then spraying the catalyst slurry on the surface of the GDL that already contains WMA to form a calalyst layer,and then with proton exchange membrane（PEM）is hot pressed to produce MEA with self-humidifying ability.Experiments show that WMA with hydrophilic anode and hydrophobic cathode can effectively improve the performance of PEMFC in low humidity.We believe that this is due to the back diffusion of water generated by the cell reaction to the anode,which is bound by the hydrophilic WMA of the anode and hydrates the PEM,allowing proton transmission.Maintained to maintain a certain performance to produce a benign cycle,while the anode hydrophobic WMA accelerates the discharge of water,and cannot improve the cell performance under low humidity.Moreover,when the hydrophilic material is less than 1.5 mg cm-2,the improvement effect becomes more obvious with the increase of WMA dosage.In addition,the use of hydrophobic WMA for the cathode under high humidity can effectively alleviate the water transportation of the cell at high density,reduce or even avoid the occurrence of flooding,and improve the cell performance.